FRP is widely applied by utilizing its characteristics of light weight, high strength and high rigidity, in fields ranging from sports and leisure applications such as fishing rods, golf club shafts and the like, to industrial applications such as automobiles, aircraft and the like.
As a method for producing FRP, suitable is a method using a prepreg as an intermediate material which is obtained by impregnating resin into a reinforcer which includes filaments such as reinforced fiber and the like, as the amount of reinforced fiber contained in the prepreg is controllable and capable of being designed at a relatively high ratio.
The specific method to obtain FRP from prepreg includes a method using an autoclave as disclosed in Japanese Unexamined Patent Application, First Publication, No. Hei-10-128778, a method using a vacuum bag as disclosed in Japanese Unexamined Patent Application, First Publication, No. 2002-159613, and a compression molding method as disclosed in Japanese Unexamined Patent Application, First Publication, No. Hei-10-95048.
Any of these methods, however, required time of about 2 to 6 hours under a condition of about 160° C. until completing curing processing such as from layering the prepreg, subjecting the layered prepreg to the intended shape to thermally cure; that is, high temperatures and long treatment times are required.
In order to make mass production of products possible, a molding is desired which can be carried out at a relatively low temperature in a range of about 100 to 130° C. in a short time ranging from a few minutes to several tens of minutes. A method to achieve the purpose includes use of epoxy resin compositions which commence curing thereof with little thermal energy, to shorten the time until the epoxy resin compositions complete curing thereof. However, if reaction activity is too high, this is dangerous due to the curing reaction running out of control. On the other hand, if conventionally used curing agents are applied, the increased amount of agents used may decrease mechanical properties. Moreover, such an epoxy resin composition is short in usable period thereof and may even cure in a few days preservation at room temperature. Thus, development of an epoxy resin composition having preferable reactivity, is expected.
From the consideration points of conditions suitably required for a prepreg, the following conditions are mentioned.
To be excellent in handling ability such as favorable tackiness (degree of stickiness) at around room temperature, appropriate draping ability (flexibility) and the like.
To keep handling ability for a long time, that is, to achieve long life at around room temperature, and a molded FRP to be excellent in mechanical property and thermal property thereof.
Prepregs which impregnate matrix resin such as an epoxy resin composition and the like in reinforced fibers, and are widely used as an intermediate material of fiber-reinforced composite materials can be used in various fields. Excellence in molding ability thereof is particularly required when being used for industrial applications described above.
At present, conventional prepregs need about 1 hour for thermal curing, therefore as aforementioned, if including times for temperature rising and lowering, even though dependent on conditions, the entire required time is about 2 or 3 to 6 hours in one cycle. This is an extremely long time, and one of the reasons for increased molding costs.
However, if required heating times were shortened, problems such as setting molding temperature extremely high may arise because life of the prepreg at around room temperature would be shortened. Development of a thermosetting resin composition which provides excellent properties for a prepreg, is desired.
SMC, properties of prepregs, and FRP plates are described below.
As materials used for FRP other than the prepregs, molding materials such as a sheet molding compound (hereinafter referred to as SMC) and the like are often used for molding. In producing FRP, employment of a prepreg including a substantially continuous reinforced fiber drawn and arranged in one direction (hereinafter referred to as UD prepreg), a woven prepreg or the like, is advantageous particularly in terms of strength of FRP, in comparison to employing SMC which requires much improvement as mentioned hereinafter.
The prepregs currently used, however, require additional improvements to obtain further excellent FRP with high efficiency.
As FRP plates being excellent in corrosion resistance, applications have been tried for shell plates of transport machinery including automobiles and various industrial apparatuses. For example, an FRP plate called SMC is widely used in shell plates such as bonnets, fenders and the like for automobiles.
SMC (for example, refer to Japanese Unexamined Patent Application, First Publication, No. Hei-6-286008) is a slurry like intermediate material wherein a reinforced fiber of a staple fiber of a carbon fiber or a glass fiber is mixed, with polyester resins and the like. The intermediate material is subjected to heating and to high pressure pressing (usually above 50 kg/cm2 or more) in a mold to shape base plates for a shell plate. Then, the base plates are ground by sandpaper or a file to make surfaces thereof flat and smooth, followed by color painting to form, for example, FRP shell plates for automobiles.
Since the shell plate of SMC includes reinforced fiber of staple fiber (non-continuous fiber), rigidity thereof is less than in the case of continuous fiber employed (not only because of reinforced fiber being staple fiber, but also 70 GPa of elastic modulus of glass being one-third of 210 GPa of elastic modulus of steel). Consequently, plate thickness of the shell plate becomes thicker than that of a metal shell plate, resulting in that the weight is not necessarily lighter than that of a metal shell plate; and if weight could be saved, the savings are often restricted to a small range. Furthermore, the shell plate made of SMC is easily perforated by local impact such as being struck by flying objects because of SMC employing non-continuous fiber; the local impact being important as regards required strength characteristics to the shell plate besides rigidity. Therefore, shell plates used for outdoors such as for transport machinery, must employ protection for impact resistance by, for example, an increase in thickness thereof, layering rubber or the like. Thus, a shell plate made of SMC does not work as a light weight shell plate able to replace a metal shell plate in terms of weight, that is, is not an environmentally friendly shell plate for transport machinery.
The most common reason for shell plates made of SMC being practically used, is that the base plate thereof before grinding treatment obtains nearly uniform surface quality due to the staple fiber thereof randomly (almost uniformly) distributed. When continuous fiber is used, due to unevenness or thickness irregularity caused by non-uniform fiber distribution, fiber meandering or undulating, crossing over of fibers themselves, or the like; unevenness of the base plate surface becomes larger than in the case of using a staple fiber. Therefore, in this case, the following problems arise:
1) Heavy labor necessary for grinding work
2) Grinding continuous reinforced fibers off in grinding work, resulting in mechanical and functional properties as a shell plate being further reduced.
On the other hand, it is preferable that continuous fiber can provide FRP having higher properties in terms of rigidity and strength, and lightness in weight. Although forms of continuous fiber are of a great variety such as unidirectional prepreg, fabric, three dimensional fabric and the like, none of them have yet to be implemented.
Alternatively, members including continuous fiber of reinforced fiber have been studied. Examples thereof include a member obtained by which prepregs including a unidirectionally continuous fiber and a resin are layered on a mold, and then it is cured by autoclave and the like; and a member obtained by which a pre-form such as fabric and the like is set in a mold, and then resin is injected thereto, that is, RTM (resin transfer molding) and the like. However, because of the intrinsic problems of continuous fiber, that is, unevenness or thickness irregularity due to fiber meandering or undulating, crossing over of fibers themselves; surface quality of the resultant member is low, therefore these have not yet been practically applied to shell plates of transport machinery such as automobiles and the like.
To improve surface quality, a coating method called gel coating is applied. The gel coating method (refer to Japanese Unexamined Patent Application, First Publication, No. Hei-11-171942) is a method wherein resin materials such as polyesters and the like can be used on the surface of a shell plate, are previously coated on the inner face of a mold to form a coating layer, followed by disposing a base material of reinforced fiber on the coating and then closing the mold; thereafter, resins are injected to cure, followed by stripping off to transfer the coating on a surface of the FRP shell plate. This method is industrially advantageous due to elimination of surface grinding work or painting. However, when being thermally cured, deformation such as warpage of the whole molding arises due to the difference in coefficients of linear expansion between the FRP and the gel coating layer. Therefore, this method is not suitable for a shell plate which requires accuracy, and is also not suitable for a shell plate due to development of cracks or wrinkles on the gel coating layer.
Furthermore, because the surface of FRP including continuous fiber as a reinforced fiber has unevenness, a thickness of the gel coating layer is at least 200 microns which is thicker than painting film when being painted. This results not only in increased weight but also the presence of drawbacks such as the gel coating layer cracking or exfoliating when the shell plate is distorted by outer forces; therefore, it is not suitable for a shell plate.
The cracks or exfoliation of the gel coating layer, in the case of the gel coating layer FRP used outdoors, may negate the advantages of FRP such as lightness in weight and durability due to water penetration into the FRP such as rain water and the like. Moreover, the gel coating is restricted in color selection in comparison to painting, and it is impossible to express appearance with a metallic feeling or fashionability. This causes problems in that the gel coating cannot be applied to a shell plate which requires matching color thereof to that of other members, such as a shell plate for automobiles, because of the reducing value of the whole product caused by color mismatching. There may be a possibility of providing painting on the gel coating layer, this case causes another penalty such as further increases in weight and/or cost.
Another attempt to improve surface quality has been studied by adjusting a cover factor of a carbon fiber fabric used as reinforced fiber (refer to Japanese Unexamined Patent Application, First Publication, No. 2001-322179).
However, it is difficult to keep the cover factor in a preferable range, because the woven carbon fiber passing through various processes after being woven such as processing to an intermediate material, process of cutting, layering and pre-forming, and molding to FRP. Although the cover factor can be kept in a preferable range by restricting movement of a carbon fiber with filler, it causes a disadvantage due to extreme difficulty in obtaining an FRP having a curved face shape due to the carbon fiber being restricted.
As described above, because of so few examples that prepregs, that is, FRP plates applying continuous fiber as a reinforced fiber, being practically used particularly for shell plates, structure and quantitative indication of surface quality have not yet been established for FRP which is considered for practical application.
The coefficient of linear expansion of FRP in the thickness direction thereof is larger than that of metal. If surface smoothness is poor, rain water is retained due to deformation caused by temperature change, and causes a lens effect for light such as ultraviolet, then irregular degradation on painting develops, resulting in a macule pattern on FRP.
It has been found that surface quality of shell plates, in addition to product value or long term durability as described above, renders significant effect on fluid resistance for air and water. Therefore, the necessity for surface quality improvement, for the purpose of energy savings, is required for not only automobiles but also all moving transport machinery such as tram cars, small planes, boats, ships and the like. In general, when a shell plate is made of FRP to save weight, due to the elastic modulus of FRP being smaller than that of metal; the shell plate deforms against air resistance generated during high speed movement of transport machinery and fluid resistance changes considerably. From this point of view, an independent criteria, which is different from that of metal, should be established for the surface of FRP plates.
Returning again, for practical use of an FRP plate applying continuous fiber, establishment of total technologies suitable for FRP plates such as structure, material, and quantitative indication of surface quality, is urgently desired.
Method for producing is described below.
The known art to obtain FRP from molding material, as described above, are a method using autoclave, a method using a vacuum bag, a compression molding method and the like. Of these, the compression molding method is preferable to mass production of FRP having good appearance and high strength due to the molding time thereof being relatively short in comparison to that of a method using autoclave and a method using a vacuum bag. This method also has the advantage of a complex-shaped FRP being easily producible due to easy mold machining.
However, when producing FRP by a compression molding method employing a molding material including continuous reinforced fiber as a reinforcer, a less viscous resin turbulently flows in and on FRP by applied pressure. The turbulent flow disturbs alignment of therein forced fiber, resulting in a so-called bowed filling. The bowed filling on a surface deteriorates the design, and the bowed filling inside causes disturbance of reinforced fiber at the part, resulting in a decrease in mechanical property of FRP. For this reason, production of FRP by the compression molding method has been limited to an FRP using SMC as disclosed in Japanese Unexamined Patent Application, First Publication, No. Hei-10-95048.